In one type of known system for reducing oxides of nitrogen in the exhaust of internal combustion engines (deNOx), it is known to have a pair of NOx adsorbent traps, such as with a barium carbonate adsorbent, one of which receives exhaust gas while the other receives desorbent (regeneration) gas, typically hydrogen containing reformate from a reformer such as a catalytic partial oxidizer (CPO), a non-catalytic, homogenous partial oxidizer (POx), or an autothermal reformer (ATR). To achieve this, there typically are four high temperature valves, two of which control the flow of exhaust to one of the traps or the other, and two of which control the flow of desorbent gas to the alternative trap.
Heretofore, such high temperature exhaust valves have been unable to operate without leakage. Typically on the order of 5% of the total engine exhaust will flow through the wrong adsorption bed during the adsorption period. Because there may be up to 15% oxygen in the engine exhaust, which oxygen will react with the hydrogen and carbon monoxide in the regeneration gas, a significant amount of regeneration gas is wastefully consumed by being combined with oxygen due to the leaks in the valves, which in turn wastes significant fuel. The reaction of the oxygen with hydrogen and CO of the desorbent gas will cause a rise in temperature which could deactivate the NOx adsorption bed catalyst.
High temperature valves required in controlling the exhaust and regeneration gas of a deNOx system are expensive and bulky.